Cardiac Muscle

Consist of two kinds of muscle cells

1. Contractile cells: 99%

Atrial & Ventricular Muscles

(Myocardium)

2. Conducting or Autorhythmic cells:

1%

Impulse generating and conducting system of heart

Myocardium

• Consists of interlacing bundles of Y shaped muscle cells, spirally arranged around the circumference of the heart

• Branching fibers

• Interconnected by intercalated discs

Two types of ‘Membrane Junctions’

– Desmosmes

– Gap Junctions

Has actin and myosin filaments

• Has low resistance

(intercalated disks (1/400 the resistance of cell membrane)

Atrial and ventricular “Syncytium”

• No gap junctions between Atrial & Ventricular “Syncytium”

• Abundant mitochondria (Up to 40% of cell volume)

• Heart depends on aerobic energy

• Cardiac muscle has abundant Myoglobin

1) To store O2

2) To facilitate O2 transport to mitochondria

• No New cardiac muscle is produced after infancy

• The muscle ‘Hypertrophies’ (↑size of muscle fibers)

• Actin and Myosin filaments in ‘SARCOMERE’

• Sarcotubular system

Action Potential

• Resting membrane potential (RMP) of cardiac

muscle is -85 to -95 mv

• Action potential (AP) is 105 mv

• Plateau lasts ~ 0.2 – 0.3 sec in ventricular muscle

Phase 1  Depolarization (Rapid)

Voltage gated sodium (Fast) channels. ↑PNa

then,

↓PNa+, ↑PCa++, ↑PK+ (K+ leak channels)

• Phase 2: Plateau (sustained depolarization)

PCa++ (in) = PK+ (out)

↑PCa++ (Slow Ca++-Na+ channels or L-type Ca++ channels)

• Phase 3: Repolarization

(PK+ > PCa++)

Voltage gated K+ Channels ↑PK+

• Phase 4: Restoration of RMP

K+ Leak channels, Na+-K+ ATPase

Refractory Periods: 250-300 msec (in N&M= 100 msec)

• During this time cardiac muscle cannot be re-excited

Atria = 150 msec [0.15 sec], Ventricles = 250 msec [0.25 – 0.30 sec],

Purkinje fibers = 300 msec [0.30 sec].

1. Absolute Refractory Period (ARP) = 200 msec

2. Relative Refractory Period (RRP) =50 msec

3. Supernormal Period (SNP)

 Usefulness of Prolong Refractory Period

• No tantanization

• Sustained myocardial contraction → An ‘Effective Pump’

Spreading of Action Potential

 Spreads along Sarcotubular System:

a. Through circuits of current b. Gap Junctions

 Excitation – contraction coupling:

i. ‘Ca++– Pulse’ or ‘Ca++ Spark’ or Ca++ trigger (cannot initiate

contraction alone)

ii. Ca++ induced Ca++ Release = Together built higher ICF Ca++

Results of Action Potential

• Ca++ release from sarcoplasmic reticulum

• Ca++ release from T- tubules, which are large

& very important source of Ca++.

• T-tubule Ca++ depends strongly on extra

cellular Ca++ concentration

• Mucopolysaccharides bind Ca++

Properties of Myocardium

• Excitability

• Conductivity

• Contractility

• Refractory Period

• Rhythmicity

• All or None Law